We report a rare case of a non-ruptured basilar bifurcation aneurysm presenting as a third ventricular mass producing unilateral obstructive hydrocephalus. This is the first reported case of its kind. A 70 year old woman presented with a giant basilar bifurcation aneurysm in which the aneurysm protruded into the third ventricle as a mass causing unilateral left hydrocephalus. The patient gradually became disoriented and developed a right hemiparesis and global aphasia. The right vertebral artery was occluded by placing coils intravascularly followed by an endoscopic septostomy. The patient's neurological state dramatically improved immediately. One month after the septostomy, however, the aneurysm ruptured and the patient eventually died. Treatment of the hydrocephalus only was selected instead of direct surgery or an intravascular procedure on the aneurysm, which eventually ruptured. The mechanisms for the unilateral hydrocephalus and the rupture of the aneurysm are described. The treatment strategy for these lesions is also discussed.